The invention relates to a method of optically writing and reading information as a pattern of magnetic domains having directions of magnetisation which are different from their surroundings in an information layer of a record carrier, the domains being formed by means of an optical scanning beam which is focused to a diffraction-limited radiation spot. During writing the portions of the information layer which are heated by the radiation spot are subjected to the influence of a magnetic field which is oriented substantially perpendicularly to the information layer and is generated by means of a coil through which a square-wave energising current is passed which is modulated in dependence upon an information signal. Thus, first domains having a first direction of magnetisation and second domains having a second direction of magnetisation are alternately formed. Reading is effected by detecting the variation caused by the magnetic domains in the state of polarisation of the scanning beam.
The invention also relates to an apparatus for performing the method.
Such a method of recording information is disclosed in U.S. Pat. No. 4,466,004. Generally, a laser beam, for example a diode laser beam which is focused to a diffraction-limited radiation spot by means of an optical system, is used when inscribing a magneto-optical information layer. The still unwritten information layer is premagnetised in a direction perpendicular to this layer. During writing the portion of the magneto-optical material at the location of the radiation spot is heated to a given temperature, for example, the Curie temperature, so that the coercive force is reduced locally. As a result this portion can be magnetised by a relatively small external magnetic field in a direction opposite to the original direction of magnetisation. After the relevant portion of the magneto-optical information layer has cooled down, the magnetic direction of the external magnetic field is frozen, as it were, into the information layer. By moving the radiation spot and the record carrier with respect to each other and by modulating the external magnetic field, a series of magnetic domains, or information areas, having a direction of magnetisation deviating from their surroundings can be written in the information layer, the succession of the information areas in the direction of movement representing the inscribed information.
In addition to this method, which is known as magnetic field modulation, information may alternatively be inscribed by means of a constant magnetic field having a direction opposite to the original direction of magnetisation of the information layer, and a radiation beam which is pulsed in accordance with the information to be written. The last-mentioned method is known as the laser modulation method. When using the laser modulation method, the size of the information areas is determined by the size of the radiation spot. In known systems, in which the radiation spot has a half-value width of approximately 1 .mu.m, the information areas are substantially circular with a diameter of the order of 1 .mu.m. The information density is then of the order of 300,000 bits per mm.sup.2.
There is an ever increasing need of larger information densities so that more information can be stored in a record carrier of given dimensions. To this end it must be possible to write and read information areas which are smaller than those hitherto used in a magneto-optical record carrier.
In U.S. Pat. No. 4,466,004 it is proposed to provide information areas in the form of magnetic domains in a magneto-optical record carrier, which areas have a dimension in the scanning direction which is smaller than the dimension of the write radiation spot, by switching the magnetic field at a high frequency. Firstly, the area of the information layer under the radiation spot is magnetised in a direction opposite to the original direction of magnetisation of the information layer. Then, while the radiation spot is still partly above this area, the magnetic field is reversed so that the a portion of the area acquires the original direction of magnetisation again. U.S. Pat. No. 4,466,004 does not describe how the magnetic domains with their smaller dimension in the scanning direction thus obtained can be read.
Since each information bit is recorded as one information area, each information area must be read separately. This means that reading must be performed by means of a radiation spot whose dimension in the scanning direction is of the same order as the dimension in that direction of the information areas. The read-radiation spot must therefore be considerably smaller than the write-radiation spot.
Since the size of a diffraction-limited radiation spot is proportional to .lambda./NA, in which .lambda. is the wavelength of the radiation used and NA is the numerical aperture of the objective system used, the radiation spot may only be reduced by decreasing the wavelength and/or enlarging the numerical aperture. An enlargement of the numerical aperture involves a decrease in the depth of focus of the radiation beam, so that the requirements to be imposed on the focusing of the radiation beam become more stringent. Moreover, an objective system having a larger numerical aperture is more sensitive to aberrations so that stricter tolerance requirements must be imposed on the write-read apparatus. If a diode laser is to be used as a radiation source, which is necessary in a mass product which the magneto-optical write-read apparatus envisages to be, the reduction of the wavelength of the radiation beam is not a real possibility because there are no short wavelength diode lasers which yield a sufficiently high power for writing.